The Planewardens have revealed to the Silver Stars that Sev's soul hangs in the balance, unwilling to return to its body. During a shared vision experienced by some of the Silver Stars during a failed resurrection ritual for Sev, it was seen that Sev chooses to remain in a limbo state, holding on to the presence of the demon Byvv, in order to main a connection to his sister's soul, which had been taken by the traitor Samehain - now either a cervant of Byvv, or connected to the demon through some other means (the exact nature of this connection is unknown).
The Planewardens have tasked the Silver Stars with venturing into the bowels of the earth, where there was once a planar seal, holding back the demon realms. It is there that they suspect that the demon Byvv - a powerful entity - is attempting to breach into the material world. The goal is to stop Byvv, and - for the Silver Stars - to resolve Sev's soul's unwillingness to return to its body.
Helena Wellsmith - a planewarden in her own right - will serve as the guide on the journey underground. The journey is expected to be perilous, as the previously existing roads to the area of the seal were blocked by the cataclysm. The mere existence of the seal and the roads is known by the Planewardens only through their communing with otherwordly entities who predate the cataclysm. It will be up to the party to find the way below, and to decide what to do when they get there.
The Planewardens expect that the great pre-cataclysm city that once existed in the deep may have journeyman altars that would allow the party to travel back, but there is no certainty of this.